This invention relates to a terminal adapter between a data terminal equipment (DTE) unit and a data communication network, such as a integrated services digital network (ISDN). It is to be noted that the terminal adapter is usually used to match transmission rates between the data communication network and the data terminal equipment unit with each other.
According to the definition (I.112) of CCITT recommendations, ISDN is defined as a network which establishes digital connections between users and network interfaces and which can execute integrated services. This means that a wide variety of communication or information processing services can be provided by the use of a single digital network. Herein, such communication services may be telephone, data, and facsimile communication services while information processing services may be for processing various kinds of information. As known in the art, packet communication is usually executed in the ISDN.
In addition, a specification is defined in the CCITT recommendation 1,430 as regards a first layer between basic users and network interfaces. Specifically, the specification prescribes electric and physical conditions between users and the network interfaces. In other words, the recommendation is directed to a basic interface basically used in the ISDN and a first-order group interface which is for carrying out a multiplex access and a high speed access and which is considered to be used in companies or the like.
Herein, a summary will be described hereinunder only about the specification of the basic interface related to the instant invention. Namely, the specification prescribes an interface rate of 192 Kbit/s (namely, Kbps), two B channels (64 Kbps) for transmission of user information, and a single D channel (16 Kbps) for controlling a network. This shows that signalling information is transmitted through the D channel.
In the meanwhile, it is to be noted that transmission rates between the network and the data terminal equipment (DTE) unit should be matched with each other so as to carry out communication therebetween by the use of the packet communication executed in the ISDN or the like.
For example, the transmission rate on the network side of the ISDN is equal to 64 Kbps in the B channel and 16 Kbps in the D channel, as mentioned above, while the transmission rate on the data terminal equipment unit side is equal to 9.6 Kbps and is slower than that on the network side. Taking this into account, a terminal adapter is located between the network and the data terminal equipment (DTE) unit to match the transmission rates with each other and is therefore structured by a rate matching circuit which is operated in accordance with a flag stuff rate matching method prescribed in the CCITT recommendation X.31.
In any event, such a rate matching circuit in a DTE unit must include a couple of buffer memories for storing a plurality of packets sent from the network and the data terminal equipment unit, respectively.
Practically, let packet data communication is carried out in the ISDN by the use of the B channel of which a maximum packet length becomes equal to 4096 bytes or octets. In this case, each buffer memory must have a memory region for storage of a current packet and another memory region for storage of a next following packet. In other words, each buffer memory must have a memory capacity of 2.times.4096 bytes (8192 bytes). This means that a total memory capacity of the buffer memories should have a total memory capacity which is greater than 16,384 bytes.
Moreover, when several packets should be memorized in each buffer memory so as to be processed in the terminal adapter, the memory capacity should be inevitably increased with an increase of the packets to be stored in the buffer memories.